The invention relates to a method for the calibration of measuring equipment measuring the thickness of a moving web, the measuring equipment comprising at least one first sensor, at least one second sensor and at least one reference piece.
The invention further relates to measuring equipment for measuring the thickness of a moving web, the measuring equipment comprising at least one first sensor, at least one second sensor and at least one reference piece.
Current measuring equipment for measuring a thickness of a moving web, such as a paper web, most typically consists of contacting, semi-contacting or non-contacting measuring equipment. In contacting measuring equipment there are measuring members that sweep both surfaces of the moving web. The thickness of the web to be measured is obtained by measuring the distance between the measuring members by means of electromagnetic sensors, for example. When semi-contacting measuring equipment is used, the surface of the moving web is supported to a reference piece for the duration of the measurement, the distance of the reference piece from a typically optical sensor located on the other side of the web being then measured by means of an electromagnetic sensor, for example. The thickness of the web to be measured is obtained by using the optical sensor to measure its distance from the surface of the paper and by deducting this from the distance to the reference piece obtained with the electromagnetic sensor. When non-contacting measuring equipment is used, the web is not separately supported for the duration of the measurement. The thickness of the web to be measured is obtained by measuring the distances of optical sensors arranged on both sides of the web from the paper and by deducting these distances from the distance between the optical sensors measured with a third, for example electromagnetic, sensor.
A typical problem with the above measuring equipment, as well as with other similar measuring equipment in which the distance of the object to be measured from the sensors or the distance between the sensors may for some reason vary, or in which the distance of the object to be measured from the sensors is great or the measurement range is large in relation to the required measurement accuracy, is that each sensor has a different response and that the responses may change over time due to drifting of the sensors. Moreover, electronic measuring equipment is sensitive to changes in the environment and for example all factors caused by temperature and having an effect on the result and mutual positioning of the sensitive sensors could not be totally eliminated even if it were possible to stabilize the temperature around the sensors. A temperature change in fact easily causes changes particularly in the response gains of sensors and not only in their offset, the error thus being dependent on the measurement distance. This means that the measuring equipment error cannot be eliminated by correcting only the offset of the sensors but also their responses must be mutually adjusted at least at two locations in the measurement range.
US 2005/0073694 A1 discloses an example of non-contacting measuring equipment for measuring the thickness of a moving web, such as a paper web, and a calibration arrangement for the measuring equipment. For the calibration the measuring equipment is provided with a movable calibration platform. The calibration of the sensors of the measuring equipment is started by moving the measuring equipment first aside from the paper web or other similar object to be measured. Next, the actual calibration of the measuring equipment is started. To calibrate the measuring equipment, the calibration platform and a calibration sample placed onto the calibration platform are moved within the measurement range of the measuring equipment to different distances from the sensors. According to an embodiment of the solution, the measuring equipment is further provided with a specific calibration measurement device used with the view of separately measuring very accurately the real movement of the calibration surface. During the calibration the calibration surface and the reference sample placed on top of it are thus moved to different distances from the sensors. Next, the distance of the sensors on opposite sides of the calibration sample from the opposite surfaces of the calibration sample and the distance between the sensors concerned are measured. In addition, the calibration measurement device measures separately the movement of the calibration platform. On the basis of the measurements the sensors of the measuring equipment are calibrated in relation to each other. After the calibration the calibration surface is lowered and the calibration sample is moved aside for the duration of the normal measuring operation of the measuring equipment.
The disclosed solution therefore allows sensors to be calibrated with respect to each other in such a way that a change in the distance between the object to be measured and the sensors or a change in the distance between the sensors does not change substantially the measurement result provided by the measuring equipment, if the characteristics of the object to be measured remain constant. With the disclosed solution it is not, however, possible to take into account the impact of the actual measurement conditions on the operation of the measuring equipment and its sensors and on changes taking place in them.